The present invention, in some embodiments thereof, relates to novel isolated polynucleotides and polypeptides which control acidity of a plant, and, more particularly, but not exclusively, to methods of using same for modulating acidity of plants, and for marker-assisting breeding of plants having desired acidity.
It has long been recognized that the acid level of fruits is an important determinant of quality, together with the sugar and volatile components. In many cases quality is actually determined by the sugar to acid ratio, as for example for grapes and citrus. Most fruit develop a fruit acid content in the acidic range, which contributes to taste: the pH values of expressed fruit juice is generally in the acidic range of 4-5 and certain fruit, such as lemons or ripe cucumbers can reach even higher levels of acidity with pH levels below 3.
The sweet melons (Cucumis melo) are fairly unique among fleshy fruit in that they have an unusually low level of acidity, and the values for all cultivated sweet melons are in the near neutral range of about 6-7. Accordingly, sweet melons have an unusually low organic acid content. Citric acid, the major organic acid in sweet melon cultivars studied to date, contributes only about 0.2% of the fruit fresh weight. This is in contrast to ripe fruit such as strawberry, pineapple or apricot, which can contain about 5 times the amount of organic acid.
One of the useful characteristics of Cucumis melo is that there exist primitive varieties within the species that have acidic fruit with nearly 1% organic acid concentration. The trait has been studied and the inheritance determined to be controlled by a single major locus, termed So (Sour) or pH (Danin-Poleg et al., 2002, Euphytica, 125: 373-384; Burger et al., 2003, J. Amer. Soc. Hort. Sci. 128: 537-540). Sour, low pH fruit, is dominant over non-sour fruit and the evolution under domestication of the sweet melons was apparently accompanied by selection for the non-sour recessive mutant soso (Burger et al., 2003, Supra). All of the cultivated sweet, high sugar melon varieties, irrespective of the fruit group (cantaloupe, honeydew, galia, charantais) have a low acidic content while all the acidic primitive cultivated and non-cultivated varieties have low sugar content. However, there is a global need in the markets for fruit having sweet and sour taste.
Despite the importance of organic acid accumulation and metabolism in fruit, little is known regarding the pathways and their control of the large temporal and genetic differences. The complexity of the pathway, its multi-components and multi-compartmentation of the pathway makes the study of individual enzymes Sisyphean in its approach.
There also exists genetic variability for acid levels in other species, such as citrus, tomato, grape and peach varieties, wherein changes in organic acids and sugars during early stages of development of acidic and acidless citrus fruit have been described.
U.S. Pat. No. 5,476,998 describes a sour tasting Cucumis melo F1 hybrid melon, derived from the breeding line produced from a plant grown from a seed having dominant allele that produces flesh with a mean pH value below 5.4 and at least one dominant allele for expression of juicy character in the flesh.
Boualem A., et al. 2008 (Science 231: 836-838) describe a mutation in ethylene biosynthesis enzyme which leads to andromonoecy in melons.
Harel-Beja et al. 2010 (Theor. Appl. Genet, 121: 511-533) describe a genetic map of melon highly enriched with fruit quality Quantitative trait loci (QTLs) and expressed sequence tag (EST) markers including sugar and carotenoid metabolism genes.
In the flower industry, the flower color is one of the most important traits of flowers. Although cultivars of various colors have been bred using conventional breeding by crossing, it is rare that a single plant species has cultivars of all colors. The main components of flower color are a group of flavonoid compounds termed anthocyanins, the color of which depends partly on their structures. In addition, since anthocyanins are present in the vacuole of the cell, the pH of vacuoles has a great impact on the color of flowers. It is thought that the vacuole of plant cells is regulated by vacuolar proton-transporting ATPase and vacuolar proton-transporting pyrophosphatase, but the mechanism of how these proton pumps are involved in the color of flowers has not been elucidated. In addition, a sodium ion-proton antiporter exits in plant vacuoles and transports sodium ions into vacuoles, depending on the proton concentration gradient between the outside and the inside of vacuoles, whereupon protons are transported outside of vacuoles resulting in a reduced proton concentration gradient. It is believed that if the pH of vacuoles could be modified, e.g., raised, flower color could be turned blue. Representative plant species that lack blue colors include roses, chrysanthemums, carnations, gerberas and the like, which are very important cut flowers.
U.S. Pat. No. 6,803,500 discloses genes encoding proteins regulating the pH of vacuoles.
Quattrocchio F., 2006 (The Plant Cell, Vol. 18, 1274-1291) describe the identification of PH4 of Petunia, an R2R3MYB Protein, that activates vacuolar acidification through interactions with basic-helix-loop-helix transcription factors of the anthocyanin pathway.
The pH of fruit affects the post harvest quality of the fruit. Thus, reduction of pH may have positive effects on fruit storage and on inhibition of pathogenic attacks, for example in tomato paste products [Clayero, M. R. S. 2001, Acta Horticulturae, 542: 75-81]. In addition, the chemical control of enzymatic blackening or browning of cut fruit and vegetable requires the inhibition of PPO activity by adjustment of pH [Ferrar, P. H., Walker, J. R. L. Inhibition of diphenol oxidases—a comparative study, J. Food Biochem. 1996, 20, 15-30; Walker, J. R. L.; Ferrar, P. H. Diphenol oxidases, enzyme-catalyzed browning and plant disease resistance, Biotechnol. Genetic Eng. ReV. 1998, 15, 457-498].
The pH of roots and of the surrounding soil impacts on nutrient uptake from the soil. For example, Aluminum uptake is strongly increased at pH below 5 and leads to aluminum toxicity of the plant [Panda S K, et al. Aluminum stress signaling in plants. Plant Signal Behav. 2009 4:592-7]. Furthermore, with decreased soil pH, dramatic increases in heavy metal desorption from soil constituents and dissolution in soil solution were observed for Cd, Pb and Zn. In addition, a negative correlation between soil pH and heavy metal mobility and availability to plants has been well documented in numerous studies (Zeng F, 2011 The influence of pH and organic matter content in paddy soil on heavy metal availability and their uptake by rice plants. Environ Pollut. 159:84-9).
Modifying rhizosphere pH can also positively contribute to the uptake of non-toxic necessary elements making certain nutrients available to the plant [Haynes R J, and Swift R S, 1985. Effects of soil acidification on the chemical extractability of Fe, Mn, Zn and Cu and the growth and micronutrient uptake of high bush blueberry plants. Plant Soil 84, 201-212; Silber, A., Ben Yones, L. and Dori, I. (2004). Rhizosphere pH as a result of nitrogen level and NH4/NO3 ratio and its effect on Zn availability and on growth of rice flower (Ozothamnus diosmifolius). Plant Soil, 262, 205-213].
Additional background art includes Peters J. L. et al., 2003 (Trends in Plant Science, 8: 484-491); Henikoff S., et al. 2004 (TILLING. Traditional Mutagenesis Meets Functional Genomics. Plant Physiology, 135: 630-636).